My personal experience with
My personal experience with VFFs is that as the edges of the part of the soles under your toes are only a few millimeters above the ground, as soon as water starts to cumulate on it, the textile that lines the rest of your toes gets quickly soaked.
I've repeatedly experience this wearing VVFs Sprints, I presume it will be the same in the case of Classics and regular KSOs since the closest part to the ground has the same design and similar material. I've never have the "opportunity to test my KSO Treks in the rain, they may remain dry a little longer because those edges are not so close to the ground, but the sides of the toes compartments are also made of textile fabric, so I think they will get soaked if water reaches that level.
I woud recommend VVFs Flow or VVFs Flow Trek. The upper of both are totally made of neoprene which is directly sealed to the rubber soles. The latter have warmer soles however they are far more expensive and you would need to order from some European retailer. Flow Treks are much more affordable ($90 new) and widely available in US. I remember having read in a blog of a girl living in Alaska her impressions after running in VVFs Flow on snow. She could keep her feet dry but the thin soles didn't protect much from the cold, she was looking forward to Trek-like soles to keep her feet warmer in winter.
In the next few days I will capable to give my personal impression about Flow Treks, not yet about thermal isolation but at least on its "water tight" and sweat issues. I'll also check how snug the neoprene vamps feel compared to my other VFFs of equivalent size.
BTW, I usually wear my VVFs Sprints without socks. To prevent the sweated feet feeling after some hours and bad smell after a few days I put some sodium bicarbonate and a lesser amount of talcum powder in them every time I'm going to wear them. I put much more than necessary to spread the mixture with ease by just inclining the shoe back and forth, then I recover the excess that didn't get attached to the inside in a plastic bag to use it next time.
I read in the Barefoot section of the Runner's World forums that sodium bicarbonate inhibits bacterial growth and I decided to add talcum powder because it is highly hygroscopic, therefore your feet will remain dry much longer. So far it seems to handle well the sweat problem as long as I don't step in some puddle or a hidden water trap like an iron lid or loosed slab and get my shoes soaked.